I think that you will find that the brake MC is 3/4". The B16 used a Girling MC. Later cars used the ATE black cast iron pot but I think that is 3/4" too or the metric equivalent. I've seen both Tilton and Wilwood MC's used on members cars as well as cast iron Girling with a plastic pot so maybe one of those could comment. I have pictures taken at Sywell of the Tilton and Wilwood install. Wilwood is copycat Girling but the Tilton has an oblong fluid chamber.The fitting factor is needing bolts at 12 and 6 o'clock.
Basically there is no reason not to use the ATE one. There is a plastic pot that replaces that cap on the ATE MC if you want a visible check on fluid level. I don't know if Simon has these.
https://shop.nordicar.com/en-gb/volv...voir-7083.html
This maybe the same as used on VW Combi/Bus and might be cheaper from VW parts suppliers.